Deoxygenation turns the coastal Red Sea lagoons into sources of nitrous oxide

Direct measurements of dissolved N₂O concentrations, fluxes and saturation percentages undertaken for the first time in two coastal lagoons - Al-Shabab and Al-Arbaeen, along the east coast of the Red Sea, revealed the region as a significant source of N₂O to the atmosphere. The exacerbated dissolved inorganic nitrogen (DIN) from various anthropogenic sources led to substantial oxygen depletion in both the lagoons, which turned to bottom anoxia at Al-Arbaeen lagoon during the spring season. We assume that the accumulation of N₂O is caused by nitrifier-denitrification in the hypoxic/anoxic boundaries. In fact, the results indicated that oxygen-depleted bottom waters favoured denitrification when the oxygenated surface waters recorded nitrification signals. Overall, the N₂O concentration ranged from 109.4 to 788.6 nM (40.6-325.6 nM) in spring and 58.7 to 209.8 nM (35.8-89.9 nM) in winter in the Al-Arbaeen (Al-Shabab) lagoon. The N₂O flux ranged from 647.1 to 1763.2 μmol m^-2 day^-1 (85.9 to 160.2 μmol m^-2 day^-1) and 112.5 to 150.8 μmol m^-2 day^-1 (76.1 to 88.7 μmol m^-2 day^-1) in the spring and winter respectively, in the Al-Arbaeen (Al-Shabab) lagoons. The ongoing developmental activities may worsen the current situation of hypoxia and associated biogeochemical feedbacks; therefore, the present results underline the need for continuous monitoring of both lagoons to restrict more severe oxygen depletion in future.